Owners and operators of vehicles having drum brakes need to know the brake's state of adjustment and brake wear to ensure safe operation of a vehicle. Brakes that are out of adjustment may not supply a sufficient amount of stopping power or may drag causing unnecessary wear and may cause the brakes to overheat. Brakes that are worn beyond an acceptable level may not supply a sufficient amount of stopping power for safe operation of the vehicle.
Some drum brakes are actuated by an S-cam or Z-cam assembly. S-cam brakes are actuated by a pneumatically driven push rod, which rotates a cam shaft on which the S-cam is mounted. The S-cam forces a pair of brake shoes against an interior surface of a brake drum. It is known in the art to measure the angular rotation of the S-cam to determine the adjustment of the brakes and wear of the brakes. However, to employ this method calculations must be performed to convert the angular rotation of the S-cam to an estimated travel distance at the brake shoe center. This calculation differs depending on the brake size and type, as well as the effective radius of the S-cam. For example, a different calculation is required for brakes having different diameters, when brakes use a single anchor pin rather than dual anchor pins, and when the effective radius of the cam changes. The computer or system reading the output. of the S-cam type sensors must be configured to calculate the brake shoe travel of the particular brake type and size that it is monitoring.
It is known in the art to use an actuator position sensor to measure brake wear and running clearance adjustment of disc aircraft brakes. Brake wear and running clearance measurements are obtained by analyzing an output of position sensing circuit. Brake disc stack height is measured using actuator position sensors. The output of an actuator position sensor is compared by a controller to a reference brake disc stack height to provide an indication of an amount of brake wear. The displacement measured by the sensor can be used to determine a running clearance position of a reciprocating ram by subtracting a predetermined clearance value from the present displacement value.
Drum brakes for land vehicles are subject to harsh conditions. For example, drum brakes, which are continuously used, become very hot. In addition, drum brakes for land vehicles are exposed to dirt, debris and often salt.
What is needed is a drum braking system having a brake shoe proximity sensor that can be used to measure brake adjustment and brake lining wear on a variety of brake types and sizes without needing to be adjusted for the particular brake type and size being used, and which will hold up under the harsh environment in which drum brakes for land vehicles are operated.